libosmocoding: move TCH ordering functions to libosmocodec and share them

The GSM 05.03 implementation present in libosmocoding is using the RTP speech frame format for TCH coding functions. It is implemented in a way of performing 'canonical -> RTP' bit reordering in decoding functions, and 'RTP -> canonical' bit reordering in encoding functions. At the moment, the RTP format bit reordering implementation is not exposed.

This API could be used by some other projects (at least by osmo-gapk), moreover libosmocoding is already being linked against libosmocodec.